As image display devices of all kinds, such as liquid crystal display devices, plasma display devices, organic EL display devices, surface-conduction electron-emitter display (SED) devices, and CRT display devices are becoming more popular these days, a demand for an optical film, one of their main components, is also increasing. The optical film is a film with various optical functions, and used for improvement of viewing angle, optical unevenness, and other optical properties to achieve a good visual quality of the image display devices. Generally, the optical film takes a multilayer structure composed of a polymer-made base and one or more upper layers.
In most cases, a polyester film is used as the base because of its transparency, dimensional stability, chemical resistance, and low hygroscopicity. The upper layer determines the function of the optical film, and may be selected from optical function layers such as a prism layer, an anti-reflection layer, and a light scattering layer, and/or mechanical function layers such as a low refractive-index layer that shows good rub resistance. These upper layers are combined with the base according to an intended purpose, and form a variety of optical films ranging from, for example, a prism film, an anti-reflection film, and light scattering film for image display devices to an IR absorbing film, an electromagnetic wave shielding film, a toning film, anti-reflection film, anti-glare film, and a hard hard coat film for the plasma display devices.
An important factor to an excellent optical property of the optical films is high adhesive strength between the base and the upper layers. However, the adhesive strength is not easily kept high because it is susceptible to the differences in composition, a forming condition, and other parameters among the base and the upper layers. In view of this, Japanese Patent Laid-open Publication No. 2001-294826 discloses a multilayer film with high adhesion strength, which has a polyester film as the base, and an adhesion assist layer placed on the base and containing a binder of polyester.
The optical film should also give a good display performance to ensure an excellent visual quality of the image display devices. In the multilayer film, however, light beams will reflect at the boundaries of the base to the adhesive layer, the adhesive layer to the upper layer, and the upper layer to the air, and such reflected light beams interfere with each other to cause uneven luminance distribution that gives a rainbow color (hereinafter, color rainbow effect). The color rainbow effect, which significantly degrades the display performance of the multilayer film, is susceptible to the thickness variation of each layer. Especially when the upper layer has thickness variation, the reflected light beam becomes more intense at a certain thickness, and the color rainbow effect is more apparent on the multilayer film.
In view of this, Japanese Patent Laid-open Publication No. 2003-177209 discloses a manufacturing method of the optical films in which the adhesion assist layer is controlled on its refractive index and thickness to reduce the rainbow colored spots. Also, Japanese Patent Laid-open Publication No. 2004-054161 discloses a multilayer film containing metal oxide particles with a controlled particle diameter and a controlled refractive index. Furthermore, Japanese Patent Laid-open Publication No. 2005-097571 discloses a multilayer film having a waterborne coating layer of a water-soluble compound and a layer stretched in at least one direction, as an upper layer on a polyester-made base. Additionally, Japanese Patent Laid-open Publication No. 2000-111706 discloses a multilayer film having the adhesion assist layer and other upper layers adjusted to create a given difference in refractive index from a high refractive index base.
Each of the upper layers, of all the above multilayer films, contains particles to adjust the optical properties such as the refractive index of the base and the upper layers, and also the adhesion assist layer contains particles or chelate compounds. However, these particles or the chelate compounds in the adhesion assist layer sometimes separate out toward the base and the upper layer, and decrease the adhesive strength between them. Although the publication No. 2000-111706 disclose to adjust the refractive index of the polymer for the adhesion assist layer, instead of adding the particles, such a polymer is generally expensive and leads to increase the manufacturing cost.